Resistance to Dicarboximide Fungicides in a Canadian Population of Microdochium nivale

Isolates of Microdochium nivale suspected of resistance to iprodione [3-(3,5-Dichlorophenyl)-N-isopropyl-2,4-dioxoimidazolidine-1-carboxamide] were collected from a golf course in Victoria, British Columbia (BC), Canada, in Fall 2015. The 9 BC isolates and 12 other reference isolates from around the world were tested with four concentrations of iprodione: 0, 1, 10, and 100 mg mL⁻¹. The BC isolates showed a half maximal effective concentration (EC₅₀) range of 14.2 to 40.8 mg mL⁻¹ with a mean of 24.2 mg mL⁻¹, while the non-BC isolates showed an EC₅₀ range of 1.2 to 4.3 mg mL⁻¹ with a mean of 1.9 mg mL⁻¹. The yielded resistance factor (EC₅₀ resistant/EC₅₀ sensitive) of 12.7 implies that these BC isolates have been selected for reduced sensitivity to iprodione. This level of resistance may be responsible for the reduced efficacy, particularly the observed reduced interval of control, but further research is needed to assess this. A test of >100 isolates of M. nivale from a worldwide collection using a discriminatory concentration of 10 mg mL⁻¹ of iprodione showed four isolates with similar levels of reduced sensitivity, but these were the only isolates from their respective populations.